Because most fights actually invite you to use it just by the way the mechanics works, where you actually want specific burst during a window as opposed to a constant over the whole fight. I will however say that some mechanics also favor IF because a wasted RoP is a hell of a dps loss.I am still trying to figure out why people are wasting their time on using Rune Of Power, it feels wrong now. It barely sims higher then IF and mechanically is easier to not use on most to all fights.
e.g.
Skorp during Weakness
Chrono when adds spawn and fast time
Trilliax probably IF
Spellblade Aluriel when the adds spawn
Tichondrius after and during Night phase, so 2 RoPs stacked more or less
Krosus you can (on heroic at least) use it safely
Botanist can go either way, but its powerful when all 3 are up and you can grab AE off the parasites too
Augur can go either way, depends if you get picked or not for Ejections
Elisande with fast bubble is nice, stacking all 3 and hurray
Gul'dan man, with the extra action bar during 3 big adds ph1, and during all Eyes
This might change a lot with Mythic mechanics though, but as of now most encounters are prone to use RoP on, additionally ofc the majority has played with RoP since Legion so it's somewhat in muscle memory.
Even if I go IF, I still press the RoP keybind when burns happen :p